Adjacent Ocean Research Articles

Stratigraphic landscape analysis, thermochronology and the episodic development of elevated, passive continental margins

The continental margin of West Greenland is similar in many respects to other elevated, passive continental margins (EPCMs) around the world. These margins are characterised by extensive regions of low relief at elevations of 1–2 kilometres above sea level sloping gently inland, with a much steeper, oceanward decline, often termed a 'Great Escarpment', terminating at a coastal plain. Recent studies, based on integration of geological, geomorphological and thermochronological evidence, have shown that the high topography of West Greenland was formed by differential uplift and dissection of an Oligo-Miocene peneplain since the late Miocene, many millions of years after continental break-up between Greenland and North America. In contrast, many studies of other EPCMs have proposed a different style of development in which the high plateaux and the steep, oceanward decline are regarded as a direct result of rifting and continental separation. Some studies assume that the elevated regions have remained high since break-up, with the high topography continuously renewed by isostasy. Others identify the elevated plains as remnants of pre-rift landscapes. Key to understanding the development of the West Greenland margin is a new approach to the study of landforms, stratigraphic landscape analysis, in which the low-relief, high-elevation plateaux at EPCMs are interpreted as uplifted peneplains: low-relief surfaces of large extent, cutting across bedrock of different age and resistance, and originally graded to sea level. Identification of different generations of peneplain (re-exposed and epigene) from regional mapping, combined with geological constraints and thermochronology, allows definition of the evolution leading to the formation of the modern-day topography. This approach is founded particularly on results from the South Swedish Dome, which document former sea levels as base levels for the formation of peneplains. These results support the view that peneplains grade towards base level, and that in the absence of other options (e.g. widespread resistant lithologies), the most likely base level is sea level. This is particularly so at continental margins due to their proximity to the adjacent ocean. Studies in which EPCMs are interpreted as related to rifting or break-up commonly favour histories involving continuous denudation of margins following rifting, and interpretation of thermochronology data in terms of monotonic cooling histories. However, in several regions, including southern Africa, south-east Australia and eastern Brazil, geological constraints demonstrate that such scenarios are inappropriate, and an episodic development involving post-breakup subsidence and burial followed later by uplift and denudation is more realistic. Such development is also indicated by the presence in sedimentary basins adjacent to many EPCMs of major erosional unconformities within the post-breakup sedimentary section which correlate with onshore denudation episodes. The nature of the processes responsible is not yet understood, but it seems likely that plate-scale forces are required in order to explain the regional extent of the effects involved. New geodynamic models are required to explain the episodic development of EPCMs, accommodating post-breakup subsidence and burial as well as subsequent uplift and denudation, long after break-up which created the characteristic, modern-day EPCM landscapes.

Oscillating Relationship between the East Asian Winter Monsoon and ENSO

AbstractThis work investigates the interdecadal variations of the relationship between the El Niño–Southern Oscillation (ENSO) and the East Asian winter monsoon (EAWM), further explores possible mechanisms, and finally considers a recent switch in the ENSO–EAWM relationship. The 23-yr sliding correlation between the Niño-3.4 index and the EAWM index reveals an obvious low-frequency oscillation with a period of about 50 yr in the ENSO–EAWM relationship. Warm ENSO events during high-correlation periods are associated with an unusually weak East Asian trough, a positive phase of the North Pacific Oscillation (NPO), significant southerly wind anomalies along coastal East Asia, and warmer East Asian continent and adjacent oceans. However, there are no robust and significant anomalies in the EAWM-related circulation during low-correlation periods. Because of the southeastward shift of the Walker circulation, the area of anomalously high pressure in the western Pacific retreats south of 25°N, confining it to the region of the Philippine Sea. In this sense, the Pacific–East Asian teleconnection is not well established. Consequently, ENSO’s impact on the EAWM is suppressed. Additionally, the low-frequency oscillation of the ENSO–EAWM relationship might be attributable to the combined effect of the Pacific decadal oscillation (PDO) and the Atlantic multidecadal oscillation owing to their modulation on the establishment of the NPO teleconnection. The observation of two full cycles of the ENSO–EAWM relationship, a transition to negative PDO in the early 2000s and an enhancement of the Walker circulation in the late 1990s, suggests a recovery of the ENSO–EAWM relationship.

A numerical experiment on the influence of the interannual variation of sea surface temperature on terrestrial precipitation in northern Japan during the cold season

The role of sea surface temperature (SST) anomaly in modulating the terrestrial precipitation in winter around Japan was investigated using a regional atmospheric model. Large amount of snowfall occurs as the westerly winter monsoon carries abundant moisture from the Japan Sea. An experiment with realistic SST gives improved representation of terrestrial precipitation distribution compared to the reanalysis. The standard deviation of interannual variation of precipitation in the experiment was approximately 20% larger over the Kuroshio extension than that in the experiment with climatology SST, suggesting that the SST variability enhances the oceanic precipitation variability. For the role of Japan Sea, the terrestrial precipitation over the Japan Sea side (JSS) region in northern Japan was sensitive to the offshore SST anomaly through affecting moisture flux toward Japan. Since the offshore SST was clearly warmer in the 2000s relative to the 1980s, the effect of the long‐term SST variation on the terrestrial precipitation trend was examined. The experiment with realistic SST simulated the observed trend in terrestrial precipitation in the JSS region. In contrast, the precipitation trend was significantly reduced in the experiment with climatology SST. Therefore, the long‐term SST trend is an important factor for the precipitation trend in the region of Japan and the adjacent oceans where SST has significant trends. Precipitation in southern Japan facing the Pacific Ocean indicated increasing trend even without the SST trend. This suggests that the long‐term variations in extratropical cyclones are also an important factor for precipitation trends around the Kuroshio extension.

Retreat history of the East Antarctic Ice Sheet since the Last Glacial Maximum

The East Antarctic Ice Sheet (EAIS) is the largest continental ice mass on Earth, and documenting its evolution since the Last Glacial Maximum (LGM) is important for understanding its present-day and future behaviour. As part of a community effort, we review geological evidence from East Antarctica that constrains the ice sheet history throughout this period (∼30,000 years ago to present). This includes terrestrial cosmogenic nuclide dates from previously glaciated regions, 14C chronologies from glacial and post-glacial deposits onshore and on the continental shelf, and ice sheet thickness changes inferred from ice cores and continental-scale ice sheet models. We also include new 14C dates from the George V Land – Terre Adélie Coast shelf. We show that the EAIS advanced to the continental shelf margin in some parts of East Antarctica, and that the ice sheet characteristically thickened by 300–400 m near the present-day coastline at these sites. This advance was associated with the formation of low-gradient ice streams that grounded at depths of >1 km below sea level on the inner continental shelf. The Lambert/Amery system thickened by a greater amount (800 m) near its present-day grounding zone, but did not advance beyond the inner continental shelf. At other sites in coastal East Antarctica (e.g. Bunger Hills, Larsemann Hills), very little change in the ice sheet margin occurred at the LGM, perhaps because ice streams accommodated any excess ice build up, leaving adjacent, ice-free areas relatively unaffected. Evidence from nunataks indicates that the amount of ice sheet thickening diminished inland at the LGM, an observation supported by ice cores, which suggest that interior ice sheet domes were ∼100 m lower than present at this time. Ice sheet recession may have started ∼18,000 years ago in the Lambert/Amery glacial system, and by ∼14,000 years ago in Mac.Robertson Land. These early pulses of deglaciation may have been responses to abrupt sea-level rise events such as Meltwater Pulse 1a, destabilising the margins of the ice sheet. It is unlikely, however, that East Antarctica contributed more than ∼1 m of eustatic sea-level equivalent to post-glacial meltwater pulses. The majority of ice sheet recession occurred after Meltwater Pulse 1a, between ∼12,000 and ∼6000 years ago, during a period when the adjacent ocean warmed significantly. Large tracts of East Antarctica remain poorly studied, and further work is required to develop a robust understanding of the LGM ice sheet expansion, and its subsequent contraction. Further work will also allow the contribution of the EAIS to post-glacial sea-level rise, and present-day estimates of glacio-isostatic adjustment to be refined.

Changes in surfzone morphodynamics driven by multi-decadal contraction of a large ebb-tidal delta

The impact of multi-decadal, large-scale deflation (76 million m3 of sediment loss) and contraction (~ 1 km) of a 150 km2 ebb-tidal delta on hydrodynamics and sediment transport at adjacent Ocean Beach in San Francisco, CA (USA), is examined using a coupled wave and circulation model. The model is forced with representative wave and tidal conditions using recent (2005) and historic (1956) ebb-tidal delta bathymetry data sets. Comparison of the simulations indicates that along north/south trending Ocean Beach the contraction and deflation of the ebb-tidal delta have resulted in significant differences in the flow and sediment dynamics. Between 1956 and 2005 the transverse bar (the shallow attachment point of the ebb-tidal delta to the shoreline) migrated northward ~ 1 km toward the inlet while a persistent alongshore flow and transport divergence point migrated south by ~ 500 m such that these features now overlap. A reduction in tidal prism and sediment supply over the last century has resulted in a net decrease in offshore tidal current-generated sediment transport at the mouth of San Francisco Bay, and a relative increase in onshore-directed wave-driven transport toward the inlet, accounting for the observed contraction of the ebb-tidal delta. Alongshore migration of the transverse bar and alongshore flow divergence have resulted in an increasing proportion of onshore migrating sediment from the ebb-tidal delta to be transported north along the beach in 2005 versus south in 1956. The northerly migrating sediment is then trapped by Pt. Lobos, a rocky headland at the northern extreme of the beach, consistent with the observed shoreline accretion in this area. Conversely, alongshore migration of the transverse bar and divergence point has decreased the sediment supply to southern Ocean Beach, consistent with the observed erosion of the shoreline in this area. This study illustrates the utility of applying a high-resolution coupled circulation-wave model for understanding coastal response to large-scale bathymetric changes over multi-decadal timescales, common to many coastal systems adjacent to urbanized estuaries and watersheds worldwide.

Intermittent dense water outflows under variable tidal forcing in Shark Bay, Western Australia

Hydrodynamic data (time series of tidal velocities and vertical stratification) were collected during the winter of 2009 in Shark Bay, Western Australia, to document water exchange between the bay and the ocean. The net loss of freshwater through evaporation causes salinity levels in Shark Bay to be higher than the adjacent ocean, leading to its classification as an inverse estuary. The observations revealed pulses of near-bed dense water outflows (velocity ∼0.10ms−1) at weekly to fortnightly intervals, associated with periods of turbulent mixing when tidal velocities and winds were both weak. Although tidal mixing appeared to be the main control on the formation of the outflows, wind mixing during strong wind events was also sufficient to destratify the water column and interrupt the density-driven circulation. These data represent the first direct measurements of exchange flows in the entrance channels of Shark Bay and reveal a mechanism to maintain the balance of salinity as well as contribute to the exchange of material (e.g., larvae) between the bay and the ocean.

Green turtles (Chelonia mydas) of the north‐western Arabian Gulf, Kuwait: the need for conservation

Abstract This study provides the first ever baseline data on reproductive ecology and genetic identification of the endangered green turtle, Chelonia mydas, in the north‐western Arabian Gulf, Kuwait. Two offshore islands were surveyed for nesting turtles in 2004 and 2005. The findings reveal that nesting (nest numbers, nesting season, incubation period, clutch size, and hatching success) by green turtles off the Kuwait coast is comparable with that of C. mydas worldwide. At Umm Al‐Maradim Island, no turtles nested during 2005 because of excavation and dredging activities associated with the construction of a coastguard dock. Mitochondrial DNA control region sequences of 97 green turtles revealed six haplotypes, of which five have not previously been reported from rookeries in adjoining regional waters or adjacent oceans; this signifies the possibility of them being novel to the region. Prudent management approaches, represented mainly by the rehabilitation and restoration of Umm Al‐Maradim Island and the designation of both foraging and nesting habitats as marine protected areas (MPAs), should be implemented by national as well as regional policy‐makers to maintain the region's native biological diversity.Copyright © 2013 John Wiley & Sons, Ltd.

Progress and Future Directions in North American Carbon Cycle Science

The North American Carbon Program (NACP) convened its fourth biennial “All Investigators” meeting (AIM4, http://www.nacarbon.org/meeting_2013) to review progress in understanding the dynamics of the carbon cycle of North America and adjacent oceans and to chart a course for a more integrative and holistic approach to future research. The meeting was structured around the six decadal goals outlined in the new “A U.S. Carbon Cycle Science Plan” (Michalak et al., University Corporation for Atmospheric Research, 2011, available at http://www.carboncyclescience.gov) and focused on (1) diagnosis of the atmospheric carbon cycle, (2) drivers of anthropogenic emissions, (3) vulnerability of carbon stocks to change, (4) ecosystem impacts of change, (5) carbon management, and (6) decision support.

Zooplankton biomass, advection and production on the northern Gulf of Alaska shelf from simulations and field observations

Interpretation of zooplankton field observations on the Gulf of Alaska (GOA) shelf is complicated by the complex physical environment of currents, eddies and meanders which mix oceanic and coastal water masses to varying degrees. We therefore developed a lower trophic level model embedded in an ocean circulation model to examine the effects of wind, runoff and temperature on transport, biomass and production of microzooplankton and copepods on the northern GOA shelf and adjacent ocean. Mean microzooplankton biomass varied from 10–90 and 10–50mgCm−3 in the upper 50m for field measurements and simulations respectively. Mean simulated and measured copepod biomass in the upper 100m was about 1–40gCm−3. Field data indicated that Neocalanus biomass was negatively correlated with salinity in May. Simulations indicated that the negative correlation was due to elevated primary and secondary production on the shelf relative to the adjacent ocean. Floats programmed to simulate the ontogenetic vertical migrations of Neocalanus indicated that ~10 to 50% of Neocalanus originating near the shelf break spend at least some time on the shelf, depending on location. Residence time of floats on the shelf was affected primarily by wind, secondarily by runoff. Primary and secondary production in the simulations were highly correlated (r>0.9). Simulated secondary production was 10–90gCm−2y−1, with highest values in Lower Cook Inlet, around Kodiak and west of Kodiak.

Downscaling reanalysis over continental Africa with a regional model: NCEP versus ERA Interim forcing

CR Climate Research Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials CR 56:181-196 (2013) - DOI: https://doi.org/10.3354/cr01152 Downscaling reanalysis over continental Africa with a regional model: NCEP versus ERA Interim forcing Leonard M. Druyan1,2,*, Matthew Fulakeza1,2 1Center for Climate Systems Research, Columbia University, and 2NASA/Goddard Institute for Space Studies, 2880 Broadway, New York, New York 10025, USA *Email: ldruyan@giss.nasa.gov ABSTRACT: Five annual climate cycles (1998-2002) are simulated for continental Africa and adjacent oceans by a regional atmospheric model (RM3). RM3 horizontal grid spacing is 0.44° at 28 vertical levels. Each of 2 simulation ensembles is driven by lateral boundary conditions from each of 2 alternative reanalysis data sets. One simulation downscales National Center for Environmental Prediction reanalysis 2 (NCPR2) and the other the European Centre for Medium Range Weather Forecasts Interim reanalysis (ERA-I). NCPR2 data are archived at 2.5° grid spacing, while a recent version of ERA-I provides data at 0.75° spacing. ERA-I-forced simulations are recommended by the Coordinated Regional Downscaling Experiment (CORDEX). Comparisons of the 2 sets of simulations with each other and with observational evidence assess the relative performance of each downscaling system. A third simulation also uses ERA-I forcing, but degraded to the same horizontal resolution as NCPR2. RM3-simulated pentad and monthly mean precipitation data are compared to Tropical Rainfall Measuring Mission (TRMM) data, gridded at 0.5°, and RM3-simulated circulation is compared to both reanalyses. Results suggest that each downscaling system provides advantages and disadvantages relative to the other. The RM3/NCPR2 achieves a more realistic northward advance of summer monsoon rains over West Africa, but RM3/ERA-I creates the more realistic monsoon circulation. Both systems recreate some features of July-September 1999 minus 2002 precipitation differences. Degrading the resolution of ERA-I driving data unrealistically slows the monsoon circulation and considerably diminishes summer rainfall rates over West Africa. The high resolution of ERA-I data, therefore, contributes to the quality of the downscaling, but NCPR2 lateral boundary conditions nevertheless produce better simulations of some features. KEY WORDS: Regional climate models · West African monsoon · Lateral boundary conditions · Reanalysis Full text in pdf format NextCite this article as: Druyan LM, Fulakeza M (2013) Downscaling reanalysis over continental Africa with a regional model: NCEP versus ERA Interim forcing. Clim Res 56:181-196. https://doi.org/10.3354/cr01152 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in CR Vol. 56, No. 3. Online publication date: April 24, 2013 Print ISSN: 0936-577X; Online ISSN: 1616-1572 Copyright © 2013 Inter-Research.

Sediment transport in the San Francisco Bay Coastal System: An overview

The papers in this special issue feature state-of-the-art approaches to understanding the physical processes related to sediment transport and geomorphology of complex coastal–estuarine systems. Here we focus on the San Francisco Bay Coastal System, extending from the lower San Joaquin–Sacramento Delta, through the Bay, and along the adjacent outer Pacific Coast. San Francisco Bay is an urbanized estuary that is impacted by numerous anthropogenic activities common to many large estuaries, including a mining legacy, channel dredging, aggregate mining, reservoirs, freshwater diversion, watershed modifications, urban run-off, ship traffic, exotic species introductions, land reclamation, and wetland restoration. The Golden Gate strait is the sole inlet connecting the Bay to the Pacific Ocean, and serves as the conduit for a tidal flow of ~ 8 × 109 m3/day, in addition to the transport of mud, sand, biogenic material, nutrients, and pollutants. Despite this physical, biological and chemical connection, resource management and prior research have often treated the Delta, Bay and adjacent ocean as separate entities, compartmentalized by artificial geographic or political boundaries. The body of work herein presents a comprehensive analysis of system-wide behavior, extending a rich heritage of sediment transport research that dates back to the groundbreaking hydraulic mining-impact research of G.K. Gilbert in the early 20th century.

Aerosol characteristics observed in southeast Queensland and implications for cloud microphysics

[1] Regular cloud base aerosol measurements were collected by a research aircraft over a 4 month period during the wet season in southeast Queensland, near Brisbane, Australia. In situ cloud microphysical measurements were also collected in many of these clouds. Brisbane is situated on the east coast of Australia and, depending on the synoptic weather conditions, can experience influences from inland or from the adjacent ocean. In situ aerosol measurements were used to compile a climatology of the aerosol conditions in the region and a back trajectory model was run for 120 h from each measurement to determine the possible air mass influences on each measurement. The most influential factor on the measured aerosol concentrations was the time the trajectory spent over land beneath 2 km (a proxy for the boundary layer). Using this criterion, the measurements were divided into two regimes: maritime for those with minimal time histories over land and continental for the remainder. Other influential factors on the aerosol concentration were also diagnosed and quantified with a regression model, including the proximity of the trajectory to the city of Brisbane and the number of fires along the trajectory. Thermodynamic, aerosol, and cloud microphysical characteristics are presented for each regime. The maritime regime tended to have more coarse mode (larger) aerosol particles and a tail of larger drops in the cloud base droplet spectra, which could be due to nucleation on the larger aerosol particles. The city influence on maritime regime trajectories yielded enhanced nucleation (fine) particle concentrations.

Caracterizacion hidrografica del estuario del rio Valdivia, centro sur de Chile

The area between 37o and 41°S of the southeastern Pacific coast, have a great number of microtidal (tidal range less than 2 m) estuaries. One of the most important estuaries in these latitudes is the Valdivia River estuary, whose thermal and haline structure is poorly known. Thus, this work, through hydrographic measurements of temperature and salinity taken during an annual seasonal cycle and the analysis of the main forcings (tide, river flow, wind and solar radiation) explain the variability and its changes in vertical stratification. The analysis of the the thermohaline structure of the water column revealed that the estuary varied seasonally behaving like a salt-wedge estuary in winter and spring due to a higher flow of tributaries. However, in summer and autumn behaved as partially mixed due the lower river flow. In winter and spring the water column showed a temperature inversion which was associated with a large surface heat loss and subsurface advection of warm waters from the adjacent ocean to into the estuary that is not mixed with the surface due to intense stratification by salinity. The change in the estuarine salt-wedge regime to partially mixed according to the season and the presence of thermal inversion seasonal are necessary hydrographic features to implement conservation efforts of vulnerable habitat into the zone (nursery areas of juvenile species), and use of this system for aquaculture activities and marine resources management.

Effects of the Tibetan Plateau on the onsetof the summer monsoon in South Asia: The role of the air‐sea interaction

Using both a coupled atmosphere‐ocean general circulation model (GCM) and an atmospheric GCM, we investigate the effects of the Tibetan Plateau (TP) on the onset of the South Asian summer monsoon by conducting simulations with and without the TP. In the coupled GCM, the presence of the TP causes the monsoon onset to occur approximately 15 days later in the Arabian Sea (AS) and India (ID) and approximately 10 days earlier in the Bay of Bengal (BB). These changes are attributed to different atmospheric circulation patterns and different conditions within the adjacent oceans, such as the AS and the BB. When the TP is included, lower sea surface temperatures (SSTs) in the AS contribute to a stable lower atmosphere, which suppresses convection over the AS and ID in May. In contrast, low pressure over South Asia, caused by the TP, induces a southwesterly toward the BB that transports a substantial amount of water vapor to the BB. This flow results in an earlier monsoon in the BB. Without the TP, higher SSTs that are formed in the AS in May destabilize the lower atmosphere and create a depression, resulting in an earlier onset of the monsoon over the AS and ID. Consequently, the cyclonic circulation spreads abruptly to the BB, and precipitation begins to increase over the BB. Therefore, the air‐sea interaction in the adjacent ocean under the influence of the TP strongly modulates the onset of the South Asian summer monsoon. This modulation was verified by the atmospheric GCM experiments.

“North drying and south wetting” summer precipitation trend over China and its potential linkage with aerosol loading

Changes in summer precipitation over China and their linkage to aerosol loading during 1957–2008 were analyzed. Increased (decreased) summer precipitation has been observed over the Southern (Northern) China, presenting a “north drying and south wetting” pattern. Such a change in precipitation pattern is related to the decreased summertime temperature over South-central China (cool zone). The cool zone, in turn, is likely caused by the increased aerosol loading, as manifested by the increased Aerosol optical depth (AOD) over this region. The cooling effect in the cool zone together with steady warming over adjacent oceans (the South China Sea and the Western North Pacific) weaken land–sea temperature contrast, and thus the East Asia summer monsoon (EASM) circulation. The weakened EASM might restrain the rainbelt in southern China longer with less moisture penetrating further north in summer season. Our analysis suggests that the aerosol loading induced “cool zone” could contribute to the “north drying with south wetting” pattern over Eastern China by altering the intensity of EASM. Furthermore, this change in precipitation pattern is also reflected in the trend of Palmer Drought Severity Index in Eastern China, which indicates a more extensive tendency of dryness in Eastern China by considering both precipitation and temperature.

Исследования изменчивости термической структуры вод по данным многочасовых зондирований в районах Антарктических станций

In the work the results of thermal water structure investigations at many-hours hydrological stations, carrying out R/V “Academic Fedorov” in 2009-2010 are presented. Shown, that the intra-daily variability of temperature field is observed against a background of inter-daily changes, which due to displacement of boundary between cold shelf water and more warm ocean water. The water structure reconstruction is result from the active synoptic processes in atmosphere above Antarctic adjacent oceans areas.

Projected changes in tropical cyclone climatology and landfall in the Southwest Indian Ocean region under enhanced anthropogenic forcing

The conformal-cubic atmospheric model, a variable-resolution global model, is applied at high spatial resolution to perform simulations of present-day and future climate over southern Africa and over the Southwest Indian Ocean. The model is forced with the bias-corrected sea-surface temperatures and sea-ice of six coupled global climate models that contributed to Assessment Report 4 of the Intergovernmental Panel on Climate Change. All six simulations are for the period 1961–2100, under the A2 emission scenario. Projections for the latter part of the 21st century indicate a decrease in the occurrence of tropical cyclones over the Southwest Indian Ocean adjacent to southern Africa, as well as a northward shift in the preferred landfall position of these systems over the southern African subcontinent. A concurrent increase in January to March rainfall is projected for northern Mozambique and southern Tanzania, with decreases projected further south over semi-arid areas such as the Limpopo River Basin where these systems make an important contribution as main cause of widespread heavy rainfall. It is shown that the projected changes in tropical cyclone attributes and regional rainfall occur in relation to changes in larger scale atmospheric temperature, pressure and wind profiles of the southern African region and adjacent oceans.

Mid-Holocene East Asian summer monsoon strengthening: Insights from Paleoclimate Modeling Intercomparison Project (PMIP) simulations

The East Asian summer (June–July–August) monsoon (EASM) is typically thought to have been stronger during interglacial periods based on spatially sparse proxy data. On a large scale, however, whether this view is true and if so, its underlying dynamic mechanisms remain unclear. Using all pertinent experiments within the Paleoclimate Modeling Intercomparison Project (PMIP), here we present an analysis of the EASM during the mid-Holocene, 6000years ago. Supporting the paleodata, the mid-Holocene EASM, as measured by regionally averaged meridional wind at 850hPa, became stronger than the baseline period in 27 out of 28 PMIP models with a demonstrable ability to simulate the modern EASM climatology. On average, the EASM strengthened by 32% across all the models and by a larger magnitude in 23 coupled models (35%) than in five atmospheric models (20%). It is proposed that an enhanced land–sea thermal contrast, and hence sea level pressure gradient, between the East Asian continent and adjacent oceans as a result of orbital forcing was responsible for the EASM strengthening during the mid-Holocene.

Characterization and comparison of microphytoplankton biomass in the lower reaches of the Biobío River and the adjacent coastal area off Central Chile during autumn winter conditions

The Biobio River in central Chile is the third largest watershed and second largest river in Chile in terms of discharge volume. Three sampling campaigns were made in the lower reaches of the river and the adjacent coastal ocean to evaluate the influence of the river plume on the physical/chemical conditions and the abundance/biomass of microphytoplankton during autumn-winter conditions. In addition, a preliminary quantification of riverine nutrients, dissolved silica, and phytoplankton carbon flux to the adjacent ocean was also conducted. High nitrate (NO3 - ) and silicic acid (Si(OH)4) concentration (>18 and 50 µM, respectively) was observed in the lower reaches of the river during all field campaigns. Si(OH)4 was even high in surface river plume waters. Hydrographic conditions indicate that river plume waters were piled coastward, and they could drove to the south at the Arauco Gulf. In all sampling, highest microphytoplankton biomass (>5000 μgC m -3 ) was associated to the lower reaches of the river and river plume waters as they were moving southward. During autumn-winter conditions a significant flux of phytoplankton carbon and nutrients to the adjacent coastal ocean also played an important role in the high biological productivity of this coastal upwelling area. These preliminary results evidence the need to conduct large-term studies, which should consider the importance of these allochthonous carbon sources in global carbon budgets and coastal food-web models.

How well can CMIP5 simulate precipitation and its controlling processes over tropical South America?

Underestimated rainfall over Amazonia was a common problem for the Coupled Model Intercomparison Project phase 3 (CMIP3) models. We investigate whether it still exists in the CMIP phase 5 (CMIP5) models and, if so, what causes these biases? Our evaluation of historical simulations shows that some models still underestimate rainfall over Amazonia. During the dry season, both convective and large-scale precipitation is underestimated in most models. GFDL-ESM2M and IPSL notably show more pentads with no rainfall. During the wet season, large-scale precipitation is still underestimated in most models. In the dry and transition seasons, models with more realistic moisture convergence and surface evapotranspiration generally have more realistic rainfall totals. In some models, overestimates of rainfall are associated with the adjacent tropical and eastern Pacific ITCZs. However, in other models, too much surface net radiation and a resultant high Bowen ratio appears to cause underestimates of rainfall. During the transition season, low pre-seasonal latent heat, high sensible flux, and a weaker influence of cold air incursions contribute to the dry bias. About half the models can capture, but overestimate, the influences of teleconnection. Based on a simple metric, HadGEM2-ES outperforms other models especially for surface conditions and atmospheric circulation. GFDL-ESM2M has the strongest dry bias presumably due to its overestimate of moisture divergence, induced by overestimated ITCZs in adjacent oceans, and reinforced by positive feedbacks between reduced cloudiness, high Bowen ratio and suppression of rainfall during the dry season, and too weak incursions of extratropical disturbances during the transition season.